Formation Of Elliptical Galaxies Research Articles

Tracing Galaxy Evolution by Their Present-Day Luminosity Function

Abstract Galaxy luminosity functions are derived for different morphological types and various colors of galaxies, to trace the evolutionary effects which a priori should be different for void and supercluster galaxies. We also analyse how the galaxy group content changes in the large-scale environment. One of the principal results is the conclusion that the evolution of spiral galaxies is almost independent of the global environment. Meanwhile, the luminosity function of elliptical galaxies depends strongly on the environment. This shows that the global environmental density is an important factor in the formation of elliptical galaxies. The results of the present study clearly show that, except the local/group environment, the global (supercluster-void) environment plays an important role in the formation and evolution of galaxies.

ORBITAL STRUCTURE OF MERGER REMNANTS. I. EFFECT OF GAS FRACTION IN PURE DISK MERGERS

Since the violent relaxation in hierarchical merging is incomplete, elliptical galaxies retain a wealth of information about their formation pathways in their present-day orbital structure. A variety of evidence indicates that gas-rich major mergers play an important role in the formation of elliptical galaxies. We simulate 1:1 disk mergers at seven different initial gas fractions ranging from 0 to 40%, using the TreeSPH code Gadget-2. We classify the stellar orbits in each remnant and construct radial profiles of the orbital content, intrinsic shape, and orientation. The dissipationless remnants are typically prolate-triaxial, dominated by box orbits within r_c ~ 1.5Reff, and by tube orbits in their outer parts. As the gas fraction increases, the box orbits within r_c are increasingly replaced by a population of short axis tubes (z-tubes) with near zero net rotation, and the remnants become progressively more oblate and round. The long axis tube (x-tube) orbits are highly streaming and relatively insensitive to the gas fraction, implying that their angular momentum is retained from the dynamically cold initial conditions. Outside r_c, the orbital structure is essentially unchanged by the gas. The 15-20% gas remnants often display disk-like kinematically distinct cores (KDCs). These remnants show an interesting resemblance, in both their velocity maps and intrinsic orbital structure, to the KDC galaxy NGC4365 (van den Bosch et al. 2008). At 30-40% gas, the remnants are rapidly rotating, with sharp embedded disks on ~ 1Reff scales. We predict a characteristic, physically intuitive orbital structure for 1:1 disk merger remnants, with a distinct transition between 1 and 3Reff that will be readily observable with combined data from the 2D kinematics surveys SAURON and SMEAGOL.

DISSIPATIONAL VERSUS DISSIPATIONLESS GALAXY FORMATION AND THE DARK MATTER CONTENT OF GALAXIES

We examine two extreme models for the build-up of the stellar component of luminous elliptical galaxies. In one case, we assume the build-up of stars is dissipational, with centrally accreted gas radiating away its orbital and thermal energy; the dark matter halo will undergo adiabatic contraction and the central dark matter density profile will steepen. For the second model, we assume the central galaxy is assembled by a series of dissipationless mergers of stellar clumps that have formed far from the nascent galaxy. In order to be accreted, these clumps lose their orbital energy to the dark matter halo via dynamical friction, thereby heating the central dark matter and smoothing the dark matter density cusp. The central dark matter density profiles differ drastically between these models. For the isolated elliptical galaxy, NGC 4494, the central dark matter densities follow the power-laws r^(-0.2) and r^(-1.7) for the dissipational and dissipationless models, respectively. By matching the dissipational and dissipationless models to observations of the stellar component of elliptical galaxies, we examine the relative contributions of dissipational and dissipationless mergers to the formation of elliptical galaxies and look for observational tests that will distinguish between these models. Comparisons to strong lensing brightest cluster galaxies yield median M*/L_B ratios of 2.1+/-0.8 and 5.2+/-1.7 at z=0.39 for the dissipational and dissipationless models, respectively. For NGC 4494, the best-fit dissipational and dissipationless models have M*/L_B=2.97 and 3.96. Comparisons to expected stellar mass-to-light ratios from passive evolution and population syntheses appear to rule out a purely dissipational formation mechanism for the central stellar regions of giant elliptical galaxies.

JD8 - Hot Interstellar Matter in Elliptical Galaxies

The physical properties of the hot interstellar matter in elliptical galaxies are directly related with the formation and evolution of elliptical galaxies via star formation episodes, environmental effects such as stripping, infall, and mergers, and growth of super-massive black holes. The recent successful Chandra and XMM-Newton X-ray space missions have provided a large amount of high spatial/spectral resolution observational data on the hot ISM in elliptical galaxies. At the same time, theoretical studies with numerical simulations and analytical modeling of the dynamical and chemical evolution of elliptical galaxies have made a significant progress and start to predict various observable quantities.

Numerical simulations of elliptical galaxies

For the formation of elliptical galaxies, two scenarios, monolithic collapse vs. major merger, have been debated. We simulate the formation and chemodynamical evolution of 128 Es from the CDM initial fluctuations, using the GRAPE-SPH code that include star formation, supernovae feedback, and chemical enrichment. In our CDM-based scenario, galaxies form through the successive merging of subgalaxies with various masses.

Stellar ages and metallicities of nearby elliptical galaxies

Stellar ages and metallicities are crucial for understanding the formation and evolution of elliptical galaxies. However, due to the age--metallicity degeneracy, it is hard to measure these two parameters accurately with broad-band photometry. In this paper, we observed high-resolution spectra for a sample of 20 nearby elliptical galaxies (EGs) with the NAOC 2.16 m telescope, and determined stellar ages and metallicities by using the empirical population synthesis and Lick/IDS index system methods. We found that stellar ages from these two methods are consistent with each other for purely old EGs; however, stellar metallicities show a zeropoint offset of 0.5 Z⊙. Our results confirm that stellar populations in low-density environment galaxies are more diverse compared to their high-density counterparts. We also investigated the element abundance--galaxy mass relation for nearby elliptical galaxies.

Formation and evolution of dwarf elliptical galaxies - II. Spatially resolved star formation histories

We present optical VLT spectroscopy of 16 dwarf elliptical galaxies (or dEs) comparable in mass to NGC 205, and belonging to the Fornax cluster and to nearby groups of galaxies. Using ULySS and STECKMAP, we derive radial profiles of the SSP-equivalent ages, metallicities and star-formation histories. The old stellar population of the dEs, which dominates their mass, is likely coeval with that of massive ellipticals or bulges, but the star formation efficiency is lower. Important intermediate age (1-5 Gyr) populations, and frequently tails of star formation until recent times are detected. These histories are reminiscent of their lower mass dSph counterparts of the Local Group. Most galaxies (10/16) show significant metallicity gradients, with metallicity declining by 0.5 dex over one half-light radius on average. These gradients are already present in the old population. The flattened (or discy), rotating objects (6/16) have flat metallicity profiles. This may be consistent with a distinct origin for these galaxies or it may be due to their geometry. The central SSP-equivalent age varies between 1 and 6 Gyr, with the age slowly increasing with radius in the vast majority of objects. The group and cluster galaxies have similar radial gradients and star-formation histories. The strong and old metallicity gradients place important constraints on the possible formation scenarios of dEs. Numerical simulations of the formation of spherical low-mass galaxies reproduce these gradients, but they require a longer time for them to build up. A gentle depletion of the gas, by ram-pressure stripping or starvation, could drive the gas-rich, star-forming progenitors to the present dEs.

Recent star-forming activity in local elliptical galaxies

The formation and evolution of elliptical galaxies (EGs) are still an open question. In particular, recent observations suggest that EGs are not only simple spheroidal systems of old stars. In this paper, we analyse a sample of EGs selected from the Sloan Digital Sky Survey in order to study the star-forming activity in local EGs. Among these 487 ellipticals, we find that 13 EGs show unambiguous evidence of recent star formation activity betrayed by conspicuous nebular emission lines. Using the evolutionary stellar population synthesis models and Lick absorption line indices, we derive stellar ages, metallicities and α-element abundances, and thus reconstruct the star formation and chemical evolution history of the star-forming elliptical galaxies (SFEGs) in our sample. We find that SFEGs have relative younger stellar population age, higher metallicity and lower stellar mass, and that their star formation history can be well described by a recent minor and short starburst superimposed on old stellar component. We also detect 11 E+A galaxies whose stellar population properties are closer to those of quiescent (normal) ellipticals than to star-forming ones. However, from the analysis of their absorption line indices, we note that our E+A galaxies show a significant fraction of intermediate-age stellar populations, remarkably different from the quiescent galaxies. This might suggest an evolutionary link between E+As and star-forming ellipticals. Finally, we confirm the relations between age, metallicity, α-element abundance and stellar mass for local EGs.

Supermassive black holes, star formation and downsizing of elliptical galaxies

The overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass, poses a serious problem for all current models of galaxy formation. Here, we improve on the one-zone chemical evolution models for elliptical galaxies by taking into account positive feedback produced in the early stages of supermassive central black hole growth. We can account for both the observed correlation and the scatter if the observed anti-hierarchical behaviour of the AGN population couples to galaxy assembly and results in an enhancement of the star formation efficiency which is proportional to galactic mass. At low and intermediate galactic masses, however, a slower mode for star formation suffices to account for the observational properties.

Ongoing assembly of massive galaxies by major merging in large groups and clusters from the SDSS

We investigate the incidence of major mergers creating massive (Mstar > 1011M⊙) galaxies in present-day (z≤ 0.12) groups and clusters. Using a volume-limited sample of 845 groups with dark matter halo masses above 2.5 × 1013M⊙, we isolate 221 galaxy pairs with ≤1.5 r-band magnitude differences, ≤30 kpc projected separations and combined masses above 1011M⊙. We fit the r-band images of each pair as the line-of-sight projection of symmetric models and identify 38 mergers by the presence of residual asymmetric structure associated with both progenitors, such as non-concentric isophotes, broad and diffuse tidal tails and dynamical friction wakes. In other words, at the resolution and sensitivity of the Sloan Digital Sky Survey (SDSS), 16 per cent of massive major pairs in dense environments have mutual tidal interaction signatures; relying on automated searches of major pairs from the SDSS spectroscopic galaxy sample will result in missing 70 per cent of these mergers owing to spectroscopic incompleteness in high-density regions. We find that 90 per cent of these mergers are between two nearly equal-mass progenitors with red-sequence colours and centrally concentrated morphologies, in agreement with numerical simulations that predict that an important mechanism for the formation of massive elliptical galaxies is the dissipationless (gas-poor or so-called dry) major merging of spheroid-dominated galaxies. We identify seven additional massive mergers with disturbed morphologies and semiresolved double nuclei; thus, 1.5 ± 0.2 per cent of Mstar≥ 5 × 1010M⊙ galaxies in large groups are involved in the major merger assembly of massive galaxies. Mergers at the centres of these groups are more common than between two satellites, but both types are morphologically indistinguishable and we tentatively conclude that the latter are likely located at the dynamical centres of large subhaloes that have recently been accreted by their host halo. Based on reasonable assumptions, the centres of group and cluster-sized haloes are gaining stellar mass at a rate of 2–9 per cent per Gyr on average. Our results indicate that the massive end of the galaxy population continues to evolve hierarchically at a measurable level, and that massive mergers are more likely to occur in large galaxy groups than in massive clusters.

Formation of $[\alpha/{\rm Fe}]$ radial gradients in the stars of elliptical galaxies

Aims. We aim: i) to test and improve our previous models of an outside-in formation for the majority of ellipticals in the context of the SN-driven wind scenario, by means of a careful study of gas inflows/outflows; ii) to explain the observed slopes, either positive or negative, in the radial gradient of the mean stellar [α/Fe], and their apparent lack of correlation with all other observables. Methods. We present a new class of hydrodynamical simulations for the formation of single elliptical galaxies in which we implement detailed prescriptions for the chemical evolution of H, He, O and Fe. Results. We find that all the models that predict chemical properties (such as the central mass-weighted abundance ratios, the colours or the [� Fe/H� ] gradient) that lie within the observed ranges for a typical elliptical, also exhibit a variety of gradients in the [� α/Fe� ] ratio, in agreement with the observations (namely positive, null or negative). All these models undergo an outside-in formation, in the sense that star formation stops earlier in the outermost than in the innermost regions, due to the onset of a galactic wind. We find that the predicted variety of gradients in the [� α/Fe� ] ratio can be explained by physical processes generally not taken into account in simple chemical evolution models, such as radial flows coupled with different initial conditions for the galactic proto-cloud. The typical [� Z/H� ] gradients predicted by our models have a slope of –0.3 dex per decade variation in radius, consistent with the mean values of several observational samples. However, we also find a quite extreme model in which this slope is –0.5 dex per decade, thus explaining some recent data on gradients in ellipticals. Conclusions. We conclude that the history of star formation is fundamental for the creation of abundance gradients in ellipticals but that radial flows with different velocity in conjunction with the duration and efficiency of star formation in different galactic regions are responsible for the gradients in the [� α/Fe� ] ratios.

Multiple minor mergers: formation of elliptical galaxies and constraints for the growth of spiral disks

Multiple, sequential mergers are unavoidable in the hierarchical build-up picture of galaxies, in particular for the minor mergers that are frequent and highly likely to have occured several times for most present-day galaxies. However, the effect of repeated minor mergers on galactic structure and evolution has not been studied systematically so far. We present a numerical study of multiple, subsequent, minor galaxy mergers, with various mass ratios ranging from 4:1 to 50:1. The N-body simulations include gas dynamics and star formation. We study the morphological and kinematical properties of the remnants, and show that several so-called “minor” mergers can lead to the formation of elliptical-like galaxies that have global morphological and kinematical properties similar to that observed in real elliptical galaxies. The properties of these systems are compared with those of elliptical galaxies produced by the standard scenario of one single major merger. We thus show that repeated minor mergers can theoretically form elliptical galaxies without major mergers, and can be more frequent than major mergers, in particular at moderate redshift. This process must then have formed some elliptical galaxies seen today, and could in particular explain the high boxiness of massive ellipticals, and some fundamental relations observed in ellipticals. In addition, because repeated minor mergers, even at high mass ratios, destroy disks into spheroids, these results indicate that spiral galaxies cannot have grown only by a succession of minor mergers.

SAURON’s Challenge for the Major Merger Scenario of Elliptical Galaxy Formation

The intrinsic anisotropy δ and flattening of simulated merger remnants is compared with elliptical galaxies that have been observed by the SAURON collaboration, and that were analyzed using axisymmetric Schwarzschild models. Collisionless binary mergers of stellar disks and disk mergers with an additional isothermal gas component, neglecting star formation, cannot reproduce the observed trend δ = 0.55 (SAURON relationship). An excellent fit of the SAURON relationship for flattened ellipticals with ⩾ 0.25 is, however, found for merger simulations of disks with gas fractions ≥20%, including star formation and stellar energy feedback. Massive black hole feedback does not strongly affect this result. Subsequent dry merging of these merger remnants does not generate the slowly rotating SAURON ellipticals, which are characterized by low ellipticities < 0.25 and low anisotropies. These objects therefore might not have been shaped by a final major merger of either early-type galaxies or disks. We show, however, that stellar spheroids resulting from multiple, hierarchical mergers of star-bursting subunits in a cosmological context are in excellent agreement with the low ellipticities and anisotropies of the slowly rotating SAURON ellipticals and their observed trend of δ with . The numerical simulations indicate that the SAURON relation might be a result of strong violent relaxation and phase mixing of multiple, kinematically cold stellar subunits, with the angular momentum of the system determining its location on the relation.

GALAXY FORMATION: THE FIRST 109 YEARS

Progress has been made in understanding the origin of spiral galaxies, but elliptical galaxy formation continues to pose many problems. At the centres of ellipticals, one finds supermassive black holes which are occasionally fed by gas accretion from their surroundings and are visible as active galactic nuclei. Negative feedback, due to the outflow from the central black hole during its active phase, terminates the gas supply to the spheroid, thereby resulting in an old stellar population. A contemporaneous phase of positive feedback when the protogalaxy is forming could result in the highly efficient phase of early star formation required by the recent data.

The hierarchical build-up of bulges in CDM

AbstractWe investigate the hierarchical build-up of stars in bulges within the standard Λ-cold dark matter scenario. By separating the population into stars born during starbursts that accompany the formation of spheroids in major mergers (starburst component), and stars that are previously formed in discs of progenitor galaxies (quiescent component) and added to the spheroid by dynamical interaction. Our results are summarised as follows: bulges that form early have larger starburst fraction and hence should be smaller than their counter parts that form later. The quiescent fraction in bulges is an increasing function of bulge mass, becoming constant at Mq/Mbul ~ 0.8, mainly due to the infall of satellite galaxies that contribute disc stars to the bulge. Minor mergers are an order of magnitude more frequent than major mergers and must play a significant role in the evolution of bulges. Above the critical mass Mc ~ 3 × 1010 M⊙ most of the stars in the universe are in spheroids, which at high redshift are exclusively elliptical galaxies and at low redshifts partly bulges. Due to the enhanced evolution of galaxies ending up in high density environments, the starburst fraction and the surface mass densities of bulges below Mc should be enhanced with respect to field galaxies. Dissipation during the formation of massive bulges in present day early-type spirals is less important than for the formation of present day elliptical galaxies of the same mass thereby explaining the possible difference in phase-space densities between spiral galaxies and elliptical galaxies.

Model colour-magnitude diagrams of elliptical galaxies

Context. Formation and evolution of elliptical galaxies are still matters of debate. Knowledge of their star formation histories (SFHs) is essential to understand the formation mechanism. Colour-magnitude (C-M) diagrams of elliptical galaxies should provide one of the most important clues to resolve the SFHs of the elliptical galaxies. Aims. The aim of this study is to illustrate what information can be obtained from C-M diagrams of elliptical galaxies and what differences appear in theoretical C-M diagrams of different formation scenarios. Methods. Using a C-M diagram simulator which consistently solves SFH and chemical evolution, we present and discuss six examples of model C-M diagrams. The C-M diagrams can be considered a first approach to the theoretical C-M diagrams of elliptical galaxies of which formation is described as the monolithic collapse and the hierarchical scenarios. Results. We show that there are three main differences in the C-M diagrams between the two formation scenarios; the magnitude of main sequence turnoff (MSTO), horizontal branch (HB) morphology, and colour distribution of red giant branch (RGB) stars. Since a later epoch of the final star formation is assumed in the hierarchical model, the magnitude of the MSTO is brighter than that of the monolithic collapse. The C-M diagram predicted by the hierarchical scenario shows well-defined blue HB. On the other hand, the C-M diagram shows few blue HB stars in the monolithic collapse case. The colour distributions predicted by the former have bluer peaks than those by the latter. We find that this corresponds to a higher frequency of metal-poor stars in the former model and metal-poor populations are born in the progenitor galaxies. In addition to the model C-M diagrams, we discuss metallicity distributions and α/Fe abundance patterns predicted by the two scenarios.

A Supermassive Black Hole Fundamental Plane for Ellipticals

We obtain the coefficients of a new fundamental plane for supermassive black holes at the centers of elliptical galaxies, involving measured central black hole mass and photometric parameters that define the light distribution. The galaxies are tightly distributed around this mass fundamental plane, with improvement in the rms residual over those obtained from the MBH-σ and MBH-L relations. This implies a strong multidimensional link between central massive black hole formation and global photometric properties of elliptical galaxies, and provides an improved estimate of black hole mass from galaxy data.

The Importance of Dry and Wet Merging on the Formation and Evolution of Elliptical Galaxies

With the aid of a simple yet robust approach we investigate the influence of dissipationless and dissipative merging on galaxy structure, and the consequent effects on the scaling laws followed by elliptical galaxies. Our results suggest that ellipticals cannot be originated by parabolic merging of low mass spheroids only, even in presence of substantial gas dissipation. However, we also found that scaling laws such as the Faber-Jackson, Kormendy, Fundamental Plane, and the Mbh -sigma relations, when considered over the whole mass range spanned by ellipticals in the local universe, are robust against merging. We conclude that galaxy scaling laws, possibly established at high redshift by the fast collapse in pre-existing dark matter halos of gas rich and clumpy stellar distributions, are compatible with a (small) number of galaxy mergers at lower redshift.

Properties of luminous red galaxies in the Sloan Digital Sky Survey

We perform population-synthesis modelling of a magnitude-limited sample of 4391 luminous red galaxies selected from the Sloan Digital Sky Survey Data Release 4 (SDSS DR4). We fit measured spectral indices using a large library of high-resolution spectra, covering a wide range of metallicities and assuming an exponentially decaying star formation rate punctuated by bursts, to obtain median-likelihood estimates for the light-weighted age, metallicity, stellar mass and internal extinction for the galaxies. The ages lie predominantly in the range 4-10 Gyr, peaking near 6 Gyr, with metallicities in the range -0.4 < [Z/H] < 0.4, peaking at [Z/H] ≈0.2. Only a few per cent of the spectra are better fitted allowing for a burst in addition to continuous star formation. For these systems, typically one-quarter to one-third of the stars are formed in the burst. The total stellar masses of all the galaxies are confined to a very narrow range around ∼3 x 10 11 M ⊙ , consistent with the expected homogeneity of the sample. Our results broadly agree with those of previous groups using an independent population-synthesis code. We find, however, that our choice in priors results in ages 1-2 Gyr smaller, decreasing the peak formation epoch from about z=2.3 to 1.3 for the stars. To describe the distribution in measured mean metallicity of the galaxies, we develop a metal evolution model incorporating stochastic star formation quenching motivated by recent attempts to account for the apparent 'antihierarchical' formation of elliptical galaxies. Two scenarios emerge, a closed box with an effective stellar yield of 0.26, and an accreting box with an effective stellar yield of 0.10. Both scenarios require an initial mass function weighted towards massive stars. They also require characteristic star formation quenching times of about 10 8 yr, the expected lifetime of luminous quasi-stellar objects. The models predict an anticorrelation between the age and mean metallicity of the galaxies similar to that observed.

Diffuse light in Hickson compact groups: the dynamically young system HCG 44

Context. Compact groups are associations of a few galaxies in which the environment plays an important role in galaxy evolution. The low group velocity dispersion favors tidal interactions and mergers, which may bring stars from galaxies to the diffuse intragroup light. Numerical simulations of galaxy clusters in hierarchical cosmologies show that the amount of the diffuse light increases with the dynamical evolution of the cluster. Aims. We search for diffuse light in the galaxy group HCG 44 in order to determine its luminosity and luminosity fraction. Combining with literature data, we aim to constrain the dynamical status of Hickson compact groups. Methods. We use Intra Group planetary nebulae (IGPNe) as tracers of diffuse light. These are detected by the so-called on band-off band technique. Results. We found 12 emission line objects in HCG 44, none of them associated with the galaxies of the group. The absence of PNe in the elliptical galaxy, NGC 3193, implies that this galaxy is located behind the group, leaving only three spiral galaxy members in HCG 44. 6/12 emission line objects are consistent with being IGPNe in HCG 44, but are also consistent with being Lyα background galaxies. Thus we derive an upper limit to the diffuse light fraction in HCG 44 of 4.7%, corresponding to 1.06 × 10 9 L� ,B and mean surface brightness of µB = 30.04 mag arcsec −2 . We find a correlation between the fraction of elliptical galaxies and the amount of diffuse light in Hickson compact groups. Those with large fraction of diffuse light are those with large fractions in number and luminosity of E/S0 galaxies. This indicates that the diffuse light is mainly created in dynamical processes during the formation of bright elliptical galaxies in major mergers. Conclusions. We propose an evolutionary sequence for Hickson compact groups in which the amount of diffuse light increases with the dynamical evolution of the group.